A conventional rotary encoder for measuring the rotational velocity of a rotating disk includes an output optical fiber connected to a light-emitting device, such as a LED, and an input optical fiber connected to a light-receiving device, such as a phototransistor. These fibers are coupled together at their front ends, and one end surfaces of the fibers are disposed opposite to the periphery of the disk, so that an effective reflective region is formed below the junction of the fibers. The periphery of the disk is provided with light-reflecting surfaces and non-reflecting surfaces. When one of the reflecting surfaces enters the reflective region, the light-receiving device produces an output of a high level. On the other hand, when one of the non-reflecting surfaces is passing through the region, the device produces an output of a low level. The number of the output pulses of the high and low levels are counted during a certain time to determine the rotational velocity of the disk.
In such a rotary encoder, it would be convenient to obtain two output waveforms that are 90 degrees out of phase, for permitting detection of the direction of the rotation or an increase in the resolution in addition to the measurement of the velocity. More specifically, when the disk is reversed, the order in which these output waveforms are generated is reversed, thus allowing detection of the direction of the rotation. When the two output waveforms are summed, a large output level can be provided, even if the pitch between the neighboring reflecting and non-reflecting surfaces is minute. Hence, the output difference between the high level and the low level increases, permitting an increase in the resolution.